Piezoelectric thin film resonators using a Bulk Acoustic Wave (BAW) are used for filters and duplexers of wireless devices such as mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. A region in which the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region. The piezoelectric film of the piezoelectric thin film resonator generally has a negative temperature coefficient of elastic constant. Therefore, the resonant frequency of the piezoelectric thin film resonator is shifted to a low frequency side as temperature increases. As described above, in an acoustic wave device using the piezoelectric thin film resonator, the resonant frequency, the anti-resonant frequency, and the passband change due to temperature.
To suppress the frequency change due to temperature, there has been suggested piezoelectric thin film resonators using an insulating film with a temperature coefficient of elastic constant opposite in sign to that of the piezoelectric film as a temperature compensation film as disclosed in Japanese Patent Application Publication Nos. 58-137317, 2013-38471, 2013-34130, and 2013-38658. To suppress a decrease in electromechanical coupling coefficient caused by the use of the temperature compensation film, there has been suggested piezoelectric thin film resonators including short-circuited conductive films formed on the upper and lower surfaces of a temperature compensation film and piezoelectric thin film resonators including a temperature compensation film embedded in the lower electrode or the upper electrode as disclosed in Japanese Patent Application Publication No. 60-16010, US Patent Application Publication No. 2011/0266925, and U.S. Pat. No. 6,420,820. Additionally, there has been known that resonance characteristics degrade when the peripheral portion of the electrode in the resonance region is thinner than a center portion as disclosed in Japanese Patent Application Publication No. 2006-109472.
When a temperature compensation film is located in the piezoelectric film to suppress the frequency change due to temperature, the electromechanical coupling coefficient decreases. When a temperature compensation film is embedded in the lower electrode or the upper electrode, electrodes on the upper and lower surfaces of the temperature compensation film are short-circuited, and thus the peripheral portion of the lower electrode or the upper electrode in the resonance region is thinner than the center portion, and resonance characteristics degrade as disclosed in Japanese Patent Application Publication No. 2006-109472.